The present invention relates to an extruder and more particularly to the feed screw of an extruder.
In the manufacture of plastic materials, the extruder receives the plastic material in its hopper and delivers such material to the feed screw for processing. The feed screw has a feed section for advancing the material at a predetermined rate, a transition section for plasticizing or working the material to turn it from a solid to a melted product, and a metering section or torpedo section for advancing the melt to the discharge end of the extruder. The present invention is directed to the improvement of the transition section which provides for a facile means for working the solid plastic materials being processed and converts such solid plastic material with maximum efficiency into a flowable mass that is fully plasticized and one that is homogeneous in nature. Heretofore it has been thought most desirable to work plastic material by working such plastic material at the feed section and transitional section with an exposure of a given mass to the rotational force of a helical feed groove which progressively diminishes in cross-sectional area without regard for its exposure to contact with the working area between the groove and the barrel. The present invention recognizes the desirability of exposing as much unplasticized solid material as possible to a working contact between the feed groove and the working area of the extruder barrel. It is not sufficient to merely reduce the cross-sectional area of the solid mass and increase the cross-sectional area of the melt groove but rather to provide for an effective maximum exposure of the solid mass to the shearing forces of the extruder. This is accomplished by providing a pair of grooves wherein a primary groove is used to convey the solid plasic material along with a secondary groove that moves the melt along. The primary groove's depth is decreased; however, the amount of material exposed to the working or melting action of the extruder barrel surface is held at a constant with maximum exposure thereto while the secondary channel which receives the melt increases in volume to assure that all the melt is retained separate from the solid unmelted plastic material.